In my U.S. Pat. No. 5,022,045, I have noted that scores of hygrometric devices have been developed in response to a world wide need among modern societies for the indication and control of the humidity of myriad processes and locations in commerce, industry, and the sciences. I particularly stressed that nearly all of these devices have been "secondary" types which depend on non-reproducible processes such as moisture sorption by various materials. As a result, the devices have no inherent accuracy, they drift badly over a period of time, and they are not suitable for myriad demanding needs.
In my pending application I noted that only a few inherently accurate "primary" type devices based on unvarying physical phenomena have been developed. These have been characterized by high cost, high power consumption, and large sensor bulk and equipment size. Heading the short list of primary devices has been the "dew temperature" or "cold mirror" hygrometer.
My pending application describes a new type of primary optical hygrometer in which the mirror is heated instead of chilled. It is analogous to the cold mirror hygrometer but it is highly superior because of its smaller size and lower power consumption. However, in spite of its general excellence for many applications, neither it nor the cold mirror hygrometer is suited for the overall ultra-miniaturization found in ICs (integrated circuits) or VLSICs (very large scale integrated circuits). This is because substantial but variable cooling or heating of the integrated circuits which are intimately associated with the operation of the sensor mirror introduces complex non-linearity problems and radiation effects.
In response to the urgent need for miniature military radiosondes, humidity sensors to incorporate within VLSIC "packages," and other critical applications, humidity sensors have been developed by others. These depend on the change of electrical capacitance of various dielectric materials as the humidity over the materials varies. However, these sensors are of the secondary type in which the moisture (which causes the dielectric constant change in the dielectric material) is sorbed in a non-linear and non-reproducible way which shifts with time. Thus, if the sensor is accessible (as in a radiosonde) a very difficult field standardization of the sensor must be attempted before sending the balloon aloft. If the sensor is sealed into a VLSIC package to monitor its interior, there is no way to check and compensate for the sensor's drift. Thus, what has been sorely needed is a capacitance-type, primary standard humidity device which a) operates at ambient temperature, b) has a sensor which is invariant, and c) has a mode for quickly checking for electronic circuit problems.
The sensors of the present invention are unusual in that the humidity-responsive sensors generate concurrent primary optical-type and primary capacitance-type signals. Even well designed and properly manufactured integrated circuits can abruptly develop circuit problems. These can result in false humidity readouts even though the capacitance/humidity changes of these sensors are invariant. With these new sensors an ongoing monitoring of the soundness of the electronics of devices which utilize the sensors in the capacitance mode can be readily provided by simultaneously optically monitoring the sensor. For example, providing a small window in one of the capacitance electrodes will allow the sensor film to be seen. It will show a dark field/bright field shift every time the gases over the sensor reach a known, precise, invariant humidity. A readout of this humidity should appear concurrently, of course, on the display screen of the capacitance meter. If desired, for convenience the same sensor material as is used between the capacitance electrodes can be optically displayed elsewhere in the system which transports the gas across the capacitance electrodes/sensor film for measurement.
The use of spatially separated dual sensors of the same composition, one sensing optically and the other capacitively, has the additional merit of allowing one to readily optimize the thickness of each for its particular function. Thus, an optical-type, humidity-responsive sensor might be coated at a thickness of about 0.01 mm. in order to secure a brilliant readout. However, a capacitance-type sensor might be coated more thinly since the capacitance and the sensitivity of an "electrode/humidity-responsive dielectric composition/electrode" triad increases, as the thickness of the dielectric composition decreases. If dual sensors are not convenient, a very small area of a very thinly coated sensor film being monitored capacitively may be of greater thickness so that when scanned optically a brilliant optical readout results.
In my U.S. Pat. Nos. 3,776,038, 4,166,891 and 4,175,207 I describe optical type, humidity-responsive devices of great utility which cover the middle range of relative humidity. However, in computer science, electronics in general and other specialized fields, the extreme humidity ranges are of special importance because of the corrosive effect of water condensing in equipment at very high humidities and static sparks (electrostatic discharges) damaging microelectronics at very low humidities. Both in electronic capacitive-responsive devices and in direct optical readout devices the sensors of the new technology allow covering the relative humidity (RH) range down to 15% and below and 85% and above. Thus, their primary, invariant qualities allow precise monitoring of ranges which until now have been notorious for generating badly drifting signals since the sorption processes used by ordinary hygrometers become increasingly erratic at very high and very low humidities.